The present invention relates, in general, to a method for preparing a photocatalyst for hydrogen production and a method for producing hydrogen by use of the same, more particularly, to a method for preparing a CdS photocatalyst for the use of hydrogen production and to a photoreaction in which hydrogen is efficiently produced from water in the presence of the CdS photocatalyst.
In general, hydrogen is used to produce ammonia and methanol in the chemical industry. Hydrogen is also an essential material for hydrogenation in which unsaturated compounds are converted into saturated ones and for hydrotreating processes, including hydrogen addition, desulfurization, denitrogenation and demetallization. Another example for the use of hydrogen is contact hydrogenation of carbon dioxide which causes global warming. In addition, hydrogen is viewed as a pollution-free, clear energy source substituting for existing fossil fuels.
Conventional techniques for obtaining hydrogen include extraction from fossil fuels, such as naphtha, modification of natural gas, reaction of vapor with iron at a high temperature, reaction of water with alkaline metal, electrolysis of water, etc.
However, the said conventional methods are not economically favorable because immense heat or electric energy is required. Regarding modification of fossil fuels, the conventional methods have another disadvantage of generating a large quantity of by-products, such as carbon dioxide. In case of electrolysis, problems, such as a short electrode lifetime and generation of by-products, should be solved to purify hydrogen more easily. Thus the cost of facilities for hydrogen production is economically unfavorable due to the noted problems.
In the nature, some of hydrogen exists, in various compounds forms, particularly in inorganic forms but most of it exists in water. Only a small quantity of hydrogen exists in the atmosphere because it is of low specific gravity. It is also very difficult and economically unfavorable to purify hydrogen existing in inorganic forms.
Therefore, a method to produce hydrogen from water will be a very. meaningful technique in near future. Recently, hydrogen producing techniques have been developed in which photocatalysts are used to decompose water into hydrogen and oxygen. However, little has been published in prior art relating to photocatalysts for producing hydrogen. Representative examples are Japanese Pat. Laid-Open Publication Nos. Sho 62-191045 and Sho 63-107815 and a couple of Korean Patent applications by the present inventors as described as below.
Japanese Pat. Laid-Open Publication No. Sho 62-191045 relates to generating hydrogen from an aqueous Na2S solution in the presence of a rare earth element compound by a photolysis reaction. The rare earth element compound has an advantage of exhibiting optical activity in the range of visible light Japanese Pat. Laid-Open Publication No. Sho 63-107815 concerns a photolysis reaction in which a composite oxide of niobium and alkaline earth metal is used as a photocatalyst to generate hydrogen from a methanol solution in water. This photocatalyst likewise has an advantage of being active in the range of visible light.
However, both of the said prior arts are disadvantageous because the amount of hydrogen generated is as little as 10 ml/0.5 g hr.
Korean Pat. Application No. 95-7721 applied by the present inventors solve the above problems to some degree by suggesting a photocatalyst represented by the following formula I:
Cs(a)/KNb6O7xe2x80x83xe2x80x83I
This technique has little affect on the environment and generates hydrogen at room temperature but the oxygen-containing organic compounds is needed as hydrogen-generating promoters.
Korean Pat. Application No. 95-30416 suggests a photocatalyst represented by the following formula II:
Cs(a)H(c)/S(b)xe2x80x83xe2x80x83II
This technique has little affect on the environment and generates hydrogen without an oxygen-containing organic compounds as a hydrogen-generating promoter at room temperature, but encounters a problem with the life time and the stability of the photocatalyst. For example, when an alkali metal, such as cesium, is impregnated into a photo-carrier, the amount of generated hydrogen is increased outstandingly but the stability of the catalyst is decreased.
Similarly, Korean Pat. Application No. 96-44214 suggests a photocatalyst represented by the following formula III:
xe2x80x83Pt(A)/Zn[M(B)]Sxe2x80x83xe2x80x83III
This technique also has little affect on the environment. This compound shows not only the optical activity of photocatalyst in some degree but also the preparation is relatively simple and the stability of photocatalyst is superior. The life time of said compound is longer which depends on electron donors and reducing agents and the amount of generated hydrogen is larger than that of prior arts. When doping with Pt instead of Cs, the stability of the catalyst is improved but still the amount of generated hydrogen is not enough in the economic point of view.
Korean Pat. Application No. 98-37179 suggests a photocatalyst represented by the following formula IV:
Pt(a)/Zn[M(b)]Sxe2x80x83xe2x80x83IV
This technique also has little affect on the environment and the said photocatalyst has optical activity in some degree in the range of visible light. The preparation of the said photocatalyst is more simpler and by-products are much less produced.
To solve the above problem, Korean Pat. Application 98-37180 by present inventors suggests a photocatalyst represented by the following formula V:
m(A)/Cd[M(B)]Sxe2x80x83xe2x80x83V
The said photocatalyst shows an optical activity in the range of visible light adjusted by light filter as well as sun light. The amount of generated hydrogen is much larger and the life time of the said photocatalyst is semi-infinitive. By introducing various doping metals and promoters and other new methods, the said application solves the restricted activity to the light source and suggests more simple preparation process. Likewise, the life time of photocatalyst is also longer and the amount of generated hydrogen from water is remarkably larger than that of prior art. However, this technique shows limited hydrogen activity only to one reducing agent.
Therefore, it is an object of the present invention to overcome the previously-noted problems encountered in prior art, and to provide economical reduction system which remarkably improves the restricted activity of photocatalysts of prior art.
It is an another object of the present invention to provide that the preparation of the photocatalyst in the present invention is more simple and has little affect on the environment.
It is an another object of the present invention to provide that the photocatalyst in the present invention has an optical activity in the range of visible light adjusted by light filter as well as sun light and thus the amount of generated hydrogen is much larger.
It is an further object of the present invention to provide the life time of the photocatalyst in the present invention being semi-infinitive.
A photocatalyst in accordance with the present invention represented by the following formula VI:
xe2x80x83m(A)/Cd[M(B)]Sxe2x80x83xe2x80x83VI
wherein m represents a doped metal element as an electron acceptor selected from the group of Ni, Pd, Pt, Fe, Ru, Co or an oxidized compound of these metals; A represents a percentage by weight of m, ranging from 0.10 to 5.00; M is a promoter selected from the group consisting of V, Cr, Al, P, As, Sb and Pb; B represents mole% of M/(M+Cd), ranging from 0.001 to 20.00.
A method for preparing the said photocatalyst of formula VI, comprising the steps of: dissolving Cd-containing and M-containing compounds in water in such an amount that the mol % of M ranges from 0.001 to 20.00; adding H2S or Na2S as a reactant in the solution with stirring to precipitate Cd[M]S; washing the precipitate with water and vacuum-drying the precipitate in a nitrogen environment at a temperature of 105xcx9c150xc2x0 C. for 1.5xcx9c3.0 hours; doping a liquid m-containing compound to this precipitate in such amount that the % by weight of m ranges from 0.10 to 5.00.
Likewise prior art of present inventors, hydrogen is produced by a method in which visible light adjusted by a light filter or sun light is irradiated onto a suspension of the said photocatalyst in water to which Na2S as a electron donor and NaH2PO2 or Na2SO3 as a reducing agent have been added.
In detail, the present invention will be described as below.
In the formula VI, m represents a doped metal element as an electron acceptor selected from the group of Ni, Pd, Pt, Fe, Ru, Co or an oxidized compound of these metals; A represents a percentage by weight of m, ranging from 0.10 to 5.00. Below 0.10 % by weight, the amount of generated hydrogen is decreased, and the stability of the photocatalyst is also reduced. On the other hand, over 5.00% by weight, the amount of generated hydrogen is decreased, and the cost of production of photocatalyst is also increased.
M is a promoter selected from the group consisting of V, Cr, Al, P, As, Sb and Pb; B represents mole% of M/(M+Cd), ranging from 0.001 to 20.00. In case of less than 0.001 mole % of M, the function of photocatalyst is lost, and in case of over 20.00 mole % of M, the amount of generated hydrogen is decreased.
The appropriate molar ratio of Cd/S is from 1:0.1 to 1:2.8, and more desirably from 1:0.6 to 1:1.4. Within said molar ratio, the effectiveness of the photocatalyst is improved.
In the preparation of the said photocatalyst, if m is platinum(Pt) as a doping element, it is preferable for Pt to be illuminated with UV in a nitrogen atmosphere and doped on the Cd[M]S by sintering. More preferably, hydrogen hexachloroplatinate(IV) (H2PtCl6) is added to the Cd[M]S precipitate and irradiated with UV light in a nitrogen atmosphere to impregnate the carrier at such an amount that the value of A for Pt(A) ranges from 0.10 to 5.00. The precipitate thus obtained is washed with water until the wash water pH reaches 7, vacuum-dried at 105 to 130xc2x0 C. for 1.5 to 3.0 hours, oxidation-sintered at 300 to 400xc2x0 C. for 1.0 to 5.0 hours and then reduction-sintered at 300 to 400xc2x0 C. for 1.0 to 5.0 hours.
In case of other doping elements except Pt, the preferable preparation example of the photocatalyst comprises the steps of :adding an m-containig compound other than platinum to the Cd[M]S precipitate obtained to reach the value of m ranging from 0.10 to 5.00; adding slowly 6 or 7 drops of conc. hydrochloric acid with stirring; applying ultrasonication to the obtained slurry for 1.0 to 5.0 minutes; drying at 110 to 130xc2x0 C. for 1.5 to 3.0 hours in vacuo; oxidation-sintering at 300 to 400xc2x0 C. for 1.0 to 5.0 hours and then reduction-sintering at 300 to 400xc2x0 C. for 1.0 to 5.0 hours.
In the preparation of photocatalyst doped with Pt, the reason why it is dried and sintered at oxidation/reduction state after the pH reaches 7 is to keep electron acceptor, Pt, in pure state. As well known, when Pt in H2PtCl6 is irradiated with UV, Pt activates the surface of CdS and makes a bond with separated S to form PtS and therefore a Wurzite structure is obtained by sintering under oxidation and reduction states at a temperature of 300 to 400xc2x0 C. for 1.0 to 5.0 hours, Pt as an electron acceptor can be transferred to pure state of Pt(0). More preferably, it should be sintered at a temperature of from 320 to 390xc2x0 C. and beyond this temperature range, the lifetime and optical activity of said photocatalyst is decreased.
Examples of the Cd-containing compounds include CdCl2, CdBr2, CdI2, Cd(CH3CO2)2.xH2O, CdSO4.xH2O and Cd(NO3)2.24H2O and examples of the M-containing compounds include K2Cr2O7, Cr(NO3)3, Al(NO3)3, AlCl3, H3 PO2, NaH2 PO2, As2O5, SbCl3, MnCl3, KMnO4, Pb(NO3)2, Pb(CH3CO2)4, RuCl3, VCl3, VOSO4, VOCl3, etc. And examples of the m-containing compounds include FeCl3, H2PtCl6, RuCl3, Co(NO3)2, CoCl2, Co(CH3CO2)2, NiSO4, Ni(NO3)2, Ni(CH3CO2)2, NiCl2, NiBr2, NiI2, Pd(No3)2, etc.
In Korean Pat. Application No. 96-44214 of present inventors, etching with acid is required after the primary sintering, but in the present invention, only the procedure of drying the precipitate in vacuo in a nitrogen atmosphere is needed, so procedures for the primary sintering and etching with acid can be canceled.
However, according to the present invention hydrogen is produced by dissolving from 0.15 to 1.00 mol of Na2S as an electron donor and from 0.15 to 1.00 mol of H2PO2 or SO32xe2x88x92 as a reducing agent in primary and/or secondary distilled water or in the previously treated water, and adding the photocatalyst of the present invention thereto. Then, the thus-obtained suspension is irradiated with visible light adjusted by a light filter or sunlight with stirring at a temperature of from 5 to 85xc2x0 C. under from 0.1 up to 5 atm. to produce hydrogen from water in a high degree of efficiency.
In addition, it is an important step to keep the concentration range of electron donor and reducing agent within the noted limits. If it is below the lower limit, the amount of hydrogen generated is decreased; if it is excess, the amount of hydrogen generated can not be increased further and the optimal reaction condition is at a temperature of from 10 to 60xc2x0 C. in from a vacuum to 2 atm.
The photocatalyst of the present invention has an semi-infinitive lifetime if the electron donor and reducing agents are added repeatedly to the reaction. The reaction time of ZnS photocatalyst in prior art is only 6 to 8 hours, but, surprisingly, the reaction time of photocatalyst of the present invention is from 20 to 25 hours, which means that said photocatalyst has continuously kept its activity well.